Field of the Invention
This invention relates to an improvement of spark plugs employed in a high voltage ignition circuit of an internal combustion engine and more particularly to an improvement of a resistor in the spark plug for suppressing radio noises generated from the spark plugs.
Discussion of the Related Art
High frequency noise currents generated from a spark plug can be suppressed, for example, by means of a high frequency radio wave absorbing circuit. This circuit is made by connecting a resistor in series with a terminal metal member fitted to an end of an internal passage of the spark plug and a center electrode fitted to the other end of the internal passage.
Conventionally, a mixture of carbon, zirconia, (or alumina or magnesia) and glass baked to the internal passage of a spark plug has been known for the resistor. This resistor comprises high resistive glass and carbon (electrically conductive material) forming a current path in a zigzag shape in order to improve a noise current suppression effect. (This noise current suppression effect by means of the zigzag shape of the current path is hereinafter referred to as "a structure effect".)
The radio noise suppression effect of the conventional resistor composed of carbon, zirconia and glass, however, is not sufficient. Japanese unpublished patent application No. 224,380/1984, filed on Oct. 25, 1984 and having the same assignee as that of the present invention, proposed a spark plug having a resistor made of glass powders comprising two groups of different grain sizes, carbon and a magnetic substance.
The abovementioned spark plug is superior in a radio noise suppression effect, but it has been learned that the durability of the resistor is shortened due to reaction with carbon and the magnetic substance. It also has been learned that the reaction makes gaps around the magnetic substance and deteriorates the radio noise suppression property.